The present invention relates to sensing or determining the concentration of an analyte in a medium. More particularly, the invention relates to a sensor and to a method for making a sensor and using a sensor for sensing the concentration of an analyte, for example, a gaseous component of interest, such as oxygen, in a medium, for example, blood.
It is sometimes necessary or desirable for a physician to determine the concentration of certain gases, e.g., oxygen and carbon dioxide, in blood. This can be accomplished utilizing an optical sensor which contains an optical indicator responsive to the component of interest. The optical sensor is exposed to the blood, and excitation light is provided to the sensor so that the optical indicator can provide an optical signal indicative of a characteristic of the analyte or component of interest. For example, the optical indicator may fluoresce and provide a fluorescent optical signal as described in Lubbers et al U.S. Pat. No. RE31,897 or it may function on the principals of light absorbance as described, for example, in Fostick U.S. Pat. No. 4,041,932.
The indicator material is often physically combined with a highly gas permeable matrix material, such as a polymeric matrix material, which is used to support the indicator material as it is exposed to the analyte. By "highly analyte permeable" is meant a material which is permeable to the analyte, in particular the gas of interest, and reaches equilibrium with the surrounding environment, which is at ambient temperature and atmospheric pressure, with respect to the analyte in a period of time measured in minutes or shorter time intervals rather than hours or longer time intervals.
One problem which has presented itself in the use of such sensors is the loss of indicator material. Thus, after a period of time in use, the amount of indicator material physically combined with the matrix material is reduced and may be leached into the medium. It would be advantageous to provide a gas sensor in which the indicator had a reduced tendency to escape from the matrix material.
Another problem which is often evident is the relatively low solubility of the indicator material in the matrix material. This feature limits the amount of indicator material that can be physically combined with the matrix material. In turn, the signal produced from the indicator/matrix material combination has a reduced intensity because of the relatively reduced amount of indicator material present. Clearly it would be advantageous to include increased amounts of indicator in the matrix material.
Yafuso et al U.S. Pat. No. 4,798,738 discloses chemically attaching ionic component sensitive dye to attachment sites on a water insoluble ionic permeable hydrophilic polymer. This allows the ionic component, e.g., pH, sensor to be constructed in the size domain of an optical fiber. Gas sensors are not disclosed.